Feel your in-breath moving low into your belly, below your belly button if possible. Next allow the breath to fill the middle of your belly around the belly button. Then expand your upper belly and lower ribs. If you feel you can still add more in-breath, allow the entire abdominal area to fill. Allow the exhale to happen in whatever way is comfortable. Just make sure the exhale is equal to or longer than the inhale. Help all areas to expand and contract in the front, sides, and back.
You can also visualize your belly as a balloon being inflated with air. Imagine your belly expanding equally all the way around, starting below the belly button and working your way up to the lower ribs. Help the breath to expand fully while keeping the breath comfortable. Try to grow a bigger breath, but don’t push to the point of causing stress.
If the exercise causes stress, just go back to your regular breathing until you relax, and then return to the diaphragmatic breathing. Less is always best. Slow and gradual gets results. Help the breath grow but stay comfortable.
Do the exercises for three to ten full breaths each time. A full breath is one inhale and one exhale. These exercises can be done anytime while sitting, lying down, or walking. The trick is to breathe gently. The breath cannot be forced, but you can influence the breath to some degree. It needs to be easy and soft. This is a balancing act. If you push too hard, it will cause stress. If it is too soft, you will not grow a full breath. If possible, breathe through your nose. Always check with your doctor before beginning any new exercises.
If thoughts take you away from your focus on the breath, just let go and move your attention back to the breath. If whatever pulled you away is strong, let your attention stay on it until it lessens, and then return your focus back to your breathing.